This invention relates to broadside beam antennas formed by an array of slot radiators and, more particularly, to an array of plural columns of slot radiators extending through a thick plate of a broad wall of a waveguide, wherein phasing of electromagnetic waves is established by inclination of passages connecting input and output ports of the slots in alternating fashion for coupling with an electromagnetic wave within the waveguide.
An array of slot radiators disposed in a straight line along a wall of a waveguide is employed frequently to generate a beam of electromagnetic power. As a typical example of an array antenna composed of slot radiators, the antenna comprises a waveguide of rectangular cross section wherein the width of a broad wall is approximately double the height of a narrow wall, and wherein the slots are formed within one of the broad walls. Antennas are constructed also of a plurality of these slotted waveguides arranged side-by-side to provide a two-dimensional array of slot radiators arranged in rows and columns. To facilitate description of the antenna, a column of slot radiators is considered to be oriented in the longitudinal direction to a waveguide, in the direction of propagation of electromagnetic power, and a row of slot radiators is considered to be transverse to the waveguide. An antenna composed of a single waveguide generates a fan beam while an antenna composed of a plurality of the waveguides arranged side by side produces a beam having well-defined directivity on two dimensions.
Antennas employing slot radiators may have slots which are angled relative to a center line of the broad wall of the waveguide, or may have slots which are arranged parallel to the center line of the broad wall of the waveguide. In order to attain a desired linear polarization, and a desired illumination function of the radiating aperture of the entire antenna, the configuration of the antenna of primary interest herein is to be configured with all of the slots being parallel to each other.
A cophasal relationship among the radiations from the various slot radiators is employed for generating a broadside beam directed perpendicularly to a plane containing the plurality of waveguides. Herein, the antenna comprising the two-dimensional array of rows and columns of radiators with slots oriented in the column direction is of primary interest. One method of obtaining the cophasal relationship is to position the slot radiators in alternating offsets fashion along a centerline of each waveguide broad wall. The transverse offsetting of the slot radiators permits a coupling with a nonzero value of longitudinal component of the magnetic field of the electromagnetic wave in each of the waveguides. With a spacing of one-half guide wavelength along the direction of propagation within the waveguide, the alternation in the offsetting compensates for periodic variations in the phase of the magnetic field so as to obtain a constant value of phase in the radiated field. The waveguides are fed in phase and operate in the TE10. Since the spacing and pattern of alternation of offsetting of slot radiators is the same in each of the waveguides, good control of the radiated beam is obtained without excessive grating lobes.
However, in the event that a TE.sub.n,0 rectangular waveguide, having a single broad wall with n columns and many rows of slots is employed in lieu of the plurality of parallel slotted waveguides, then the relationship among the wave components in each of the columns changes. The phasing of the components of the wave in one column is 180 degrees out of phase with the wave components of the contiguous column. To compensate for this phasing of the wave components, the pattern of offset slot radiators of one column must be reversed from that of the contiguous columns to insure identity of phasing.
A problem arises in that the foregoing arrangement of reversed patterns of offset slot radiators introduces excessive grating lobes in addition to the desired beam. The resulting loss of antenna gain militates against the convenience of using a single broad-walled waveguide as antenna, unless the grating lobes can be eliminated.
But, to facilitate manufacture of the antenna, and to reduce the overall weight of the antenna, it would be preferable to construct the antenna of this single waveguide, wherein the broad walls are of sufficient width to form multiple columns of slot radiators within a single broad wall of a wide waveguide operating in the TE.sub.n,0 mode. This would eliminate the need for constructing the antenna with n individual waveguides joined side by side.